1. Field of the Invention
This invention relates to a television receiver whereby teletext data superimposed in a vertical flyback period can be received, compressed and displayed on a picture and also relates to a compressing method in the case of compressing the data.
2. Description of the Related Art
A character broadcasting system is generally known as a teletext. A teletext system is a system wherein, in a television broadcasting station, teletext data encoded in a predetermined horizontal scanning period in a vertical blanking period are superimposed and are transmitted together with a television signal. On the receiver side, the received teletext data are picked up, are decoded to the original character information and are displayed on a TV screen.
The television broadcasting station can transmit many teletext programs by utilizing a plurality of horizontal scanning periods. The user can first select a television channel broadcasting a program by such key input means as a remote controlled hand set and then designate the program number of the teletext program so as to receive the teletext program.
Now, in an ordinary TV receiver, when a teletext is to be received, the TV picture is switched to a teletext picture and, when a TV broadcast is to be received, the TV picture is switched to a TV broadcast picture. Therefore, both of the teletext and TV broadcast can not be simultaneously displayed on the picture. Therefore, as in the Japanese Patent Application No. 75363/l988 filed by the Applicant of the present case, by a two-picture displaying technique, either one of the broadcasts is displayed as a main picture and the other broadcast is displayed as an auxiliary picture.
FIG. 1 shows an example of a picture on which teletext data are displayed as an auxiliary picture. In FIG. 1, the teletext picture is displayed in the lower right corner of the TV broadcast picture (main picture) displayed over the whole picture. In the case of FIG. 1, the teletext picture occupies a quarter of the TV broadcast picture.
In the case of such display mode as is mentioned above, the teletext data must be compressed. In the compression, the size in the vertical direction of the picture is contracted to be half by subtracting the first or second field of the information displayed over the whole TV picture and the size in the horizontal direction of the picture is contracted by subtracting in the horizontal direction such pixel data.
There is such compressing method as a method of logically operating two pixel data with each other.
FIGS. 2 and 3 are explanatory views for explaining an example of the compressing method by the above mentioned logical operation. FIG. 2 shows a teletext picture before the compression. FIG. 3 shows a teletext picture after the compression.
As shown in FIG. 2, the teletext data are to be displayed on a TV screen in which 12 rasters and 24 pixels in the horizontal direction are arranged. The contents of the data are exemplified by "H", "F", "S", "L" and "D". In this case, the pixels forming the teletext character are hatched and are called dot pixels. The pixels forming the background part of the teletext are called blank pixels.
In order to compress the above mentioned teletext data, respectively two adjacent rows of the vertical pixel rows are made a set of an odd number row and even number row and are divided into sets of A.sub.1 and B.sub.1, A.sub.2 and B.sub.2, A.sub.12 and A.sub.12. The respective sets of pixel rows are logically operated to be added to each other. The data obtained thereby are displayed as an auxiliary picture as a compressed output. In FIG. 3, in the picture after the compression, C.sub.1, C.sub.2, and C.sub.12 correspond respectively to A.sub.1 and B.sub.1 A.sub.2 and B.sub.2, and A.sub.12 and B.sub.12.
Now, as shown in FIG. 4, there shall be considered the case that the outline of the teletext character is displayed by "blank pixels" and the background is displayed by the data representing "dot pixels". Now, if a logical addition operation preferring such dot data as are compressed in FIGS. 2 to 3 is used as a compressing method, as shown in FIG. 5A, in the picture compressed thereby, the original information will be greatly lost. That is to say, FIG. 4 is a picture (inversely displayed picture) in which the dot pixels and blank pixels are replaced with each other. When such teletext data are compressed by the same logical operation method as in FIG. 2, there will be made such picture in which the greater part of the original information has been lost as is shown in FIG. 5A.
On the contrary, if there is adopted a compressing method preferring blank pixels, that is, a method of subtracting the pixels by operating the respective logical products of A.sub.1 and B.sub.1, A.sub.2 and B.sub.2,... and A.sub.12 and B.sub.12 as shown in FIG. 5B, the information in FIG. 4 will become the same compressed picture substantially perfectly holding the original information as the picture in FIG. 3. However, if the data in FIG. 2 are compressed by the compressing method preferring the blank pixels, as shown in FIG. 6, there will be a contradiction that the original information will not be reproduced.
As mentioned above, in case the compressing rule is fixed, depending on the contents of the teletext data, the information has been properly compressed or has been lost.